The RMAH has yet to be implemented in Diablo III, but some players have already added money to their balance in preparation for its launch. Those players will be still be able to use it to pay for eligible purchases or to use in the RMAH without the added security, but adding to the balance from now on will require adding an authenticator to their accounts.
I think that preparing beforehand for the inevitable attempted account incursions is a very responsible move on Blizzard's part. This move is good for the players, as well as good for their customer service department, I'm sure.
The full announcement is after the break.